Typically, radio frequency identification (RFID) systems may be classified into an inductively coupled system and an electromagnetic wave system according to wireless connecting methods. The inductively coupled system is used in RFID systems for short distances (i.e., 1 m or less), and enables an RF signal receiver (for example, a reader) and an RF signal transmitter (for example, an RF tag) to perform wireless communications by using coil antennas.
An RF tag in the inductively coupled system is almost manually operated. In other words, all of energies required for an IC chip of the RF tag to operate are supplied by a reader. Accordingly, an antenna coil of the reader generates a strong magnetic field at its neighboring area. A part of the magnetic field emitted by the antenna coil generates an inductive voltage in a coil antenna of the RF tag, and the inductive voltage is rectified and then supplied as an energy for the IC chip.
The electromagnetic wave system is used in RFID systems for middle and long distances, and enables a reader and an RF tag to perform wireless communications by using RF antennas. Since the RF tag in the electromagnetic wave system cannot receive a sufficient amount of power for driving an IC chip from the reader, an RF tag (for example, an active type tag) including an additional battery for long-distance recognition may be used.
Each of the reader and the RF tag processes baseband data according to several digital types of coding. An RF signal generated by the RF tag may include an offset voltage. Therefore, the RF signal may be distorted by the offset voltage, and a signal-to-noise ratio (SNR) may be degraded. Consequently, reception sensitivity of the reader may decrease.